The machine tool main spindle is increasingly sped up in order to increase the machining efficiency. As a result of the speedups of the main spindle, the torque and the heat value increase in a main spindle bearing used for the main spindle. Accordingly, in order to deal with the increasing torque and the increasing heat value, a jet lubrication system or an air oil lubrication system is largely employed in lubricating the main spindle bearing.
According to the jet lubrication system, a substantial amount of oil is jetted into the bearing to accomplish both of lubrication of the bearing and cooling of the bearing simultaneously, but there is a problem in that since the stirring resistance of the lubricating oil increases (substantially proportional to the square of the speed) when the bearing is operated at high speed, the power loss of the bearing increases correspondingly, thus requiring the use of a high capacity drive motor.
Also, the air oil lubrication system is such that a transport air is mixed with a lubricating oil and the mixture of the air and the oil is jetted into the bearing through a nozzle. As a measure to reduce the stirring resistance of the oil within the bearing, it is suggested to deposit a small quantity of oil on an outer peripheral surface of an inner race of the bearing so that the oil can reach raceway surfaces of the bearing by the effect of a centrifugal force and a surface tension. (See the JP Laid-open Patent Publications No. 2001-012481 and No.
For example, in the lubricating structure disclosed in the JP Laid-open Patent Publication No. 2001-012481, which is reproduced in FIG. 8, a scoop area 50, which is an oil collecting area, is defined in one of opposite end faces of an inner race 42 of the bearing and, on the other hand, an oil supply nozzle 51 for jetting a lubricating oil towards the scoop area 50 is defined in an outer race spacer 47 that is disposed adjacent the scoop area 50. Also, the scoop area 50 is communicated with a raceway surface in the inner race 42 through a nozzle hole 52 formed in the inner race 42 so that most of the lubricating oil supplied from the oil supply nozzle 51 can enter the scoop area 50 and can then be sprayed to balls 44 through the nozzle hole 52 by the effect of a centrifugal force.
As shown in FIG. 9 showing, on an enlarged scale, a portion encompassed by the circle V in FIG. 8, a gap C of a gap size equal to or smaller than 0.2 mm is formed between one of opposite end faces of the outer race spacer 47 where the oil supply nozzle 51 is formed, and one of the opposite end faces of the inner race 42 where the scoop area 50 is formed. A portion of the lubricating oil, which is supplied from the oil supply nozzle 51, but deposits on the end face of the outer race spacer 47 without entering the scoop area 50, flows towards the end face of the inner race 42 by way of the gap C. Also, since an outer peripheral surface 50a of the inner race 42 adjacent the scoop area 50 is tapered to have a diameter increasing in a direction inwardly of the bearing and since a junction between the end face and the outer peripheral surface 50a of the inner race 42 is represented by a curved surface area 50c, the lubricating oil flowing from the oil supply nozzle 51 towards the end face of the inner race 42 is forced from the curved surface area 50c towards the outer peripheral surface 50a by the effect of a centrifugal force developed during rotation of the inner race 42 and is then supplied to an area under a retainer 45 for the balls 44.
It is to be noted that in the JP Laid-open Patent Publication No. 2001-012481 referred to above, the lubricating structure similar to that shown in described with reference to FIGS. 8 and 9, but with no nozzle hole 52 employed, is disclosed as shown in FIG. 10.
As for the air oil lubrication system, the air oil used therein has little cooling effect for the bearing. Accordingly, where the air oil lubrication system is employed, the use of a separate cooling mechanism is required. For this cooling mechanism, the cooling system is known, in which not only is a housing for the bearing cooled, but also a cooling oil is supplied towards a bore of a hollow shaft, supporting the bearing, to cool the bearing. (See, for example, the Japanese Patent No. 3084356 (the JP Laid-open Patent Publication No. H10-58278) and the JP Laid-open Patent Publications No. H07-24687 and No. H07-145819.)